"Here we show that the previously reported increase in global drought is overestimated," write Roderick and colleagues.

According to the 2007 Fourth Assessment Report (AR4) of the Intergovernmental Panel on Climate Change (IPCC), there have been more intense and longer droughts observed over wider areas since the 1970s, particularly in the tropics and subtropics.

"Increased drying linked with higher temperatures and decreased precipitation has contributed to changes in drought," the report says, quoted by Roderick and colleagues.

Roderick says such assessments of historical drought trends have relied on what is known as the Palmer Drought Severity Index (PDSI), developed in the US.

This index has also been used by government agencies, for example in the US, to estimate the severity of drought and to allocate financial aid.

But, he says, the index oversimplifies the effect of global warming by incorrectly assuming that increasing temperatures will cause more evaporation that will dry out the soil.

"Water doesn't necessarily evaporate faster as the temperature goes up. It depends on the details. And the details are all important here," says Roderick, a land surface specialist and biophysicist.

Supply and demand

Roderick says drought occurs when there is a net loss of water from the soil. This not only depends on the supply of water to the soil (from rainfall) but also on the demand for water by the atmosphere (due to evaporation).

Evaporation in turn, is influenced by more factors than temperature, says Roderick.

"What determines the evaporation rate of water predominantly is solar radiation, the humidity of the air and the wind," he says.

Roderick and colleagues reanalysed the data used in the 2007 IPCC report and found that when evaporation was assumed only to be a function of temperature global drought was shown to have been increasing.

But, when the researchers used an updated index, which included solar radiation, air humidity and wind, they found little overall global change in drought in the past 60 years.

IPCC revision

Other experts, like Dr Michael Raupach, of CSIRO Marine and Atmospheric Research, agree the PDSI is flawed.

"Anyone relying on the PDSI is likely to have overestimated drought," he says.

But they say the IPCC has already acknowledged this.

Professor Sonia Seneviratne, of the Institute for Atmospheric and Climate Science in Switzerland, says the IPCC has already changed its conclusions regarding drought, following the release of studies carried since its 2007 report.

"The 2007 IPCC Fourth Assessment Report's conclusion that the area affected by droughts was 'likely' to have increased in many regions since the 1970s had already been revised in a more recent IPCC special report on extreme events and disasters published earlier this year (the SREX Report)," says Seneviratne, in an accompanying News & Views.

Raupach says the research community has been "moving away" from the PDSI for a long time but the simple index is still commonly used, for example in agricultural communities, although he is not aware of it being used in Australia.

"It continues to be used for reasons of expediency," he says.

Role of vegetation

However, despite agreeing that the PDSI is flawed, Raupach says that there is room to advance beyond Roderick and colleagues' replacement index.

While they have improved the "demand" side of the drought equation, modifications also need to be made to the "supply" side, says Raupach.

"The way that plants supply water to the atmosphere in evaporation is determined by a whole range of biological factors as well as by these meteorological factors," he says.

Raupach says calculations that incorporate the role of vegetation in the water cycle, broadly agree that there has been little indication of increasing drought on a global scale.

But, what has been happening at the regional level, and what is projected to happen in the future?

Regional versus global

Roderick and colleagues found that while globally there has been no increase in drought, some areas of the world have been getting more droughts, and others have been getting less.

For example, says Roderick, areas such as Southeast Australia became drier between 1950 to 2008, while the Amazon has been having fewer droughts.

Raupach's research forecasts what will happen in the next 50 years in the Murray Darling Basin.

He says a projected decrease in rainfall and increase in temperature will be offset by the effect of increased efficiency of plant water use that occurs when CO2 rises.

Raupach's calculations project that water availability in the river system will decrease by between 10 and over 20 per cent over this time.

But, as ever in climate change science, he acknowledges there are "huge question marks" over all of this.